The present invention relates to a fine particle detection system for detecting the amount of fine particles such as soot in exhaust gas flowing through an exhaust pipe.
Fine particles such as soot are contained in exhaust gases of internal combustion engines (e.g. diesel engines and gasoline engines). It is common practice to purify the exhaust gas by collecting the fine particles from the exhaust gas with the use of a filter. It is also feasible to heat the filter to a high temperature that the collected fine particles can be burned off from the filter.
In the event of a deterioration or defect (e.g. breakage) in the filter, the unpurified exhaust gas is directly discharged downstream of the filter. There is thus a demand for a fine particle detection system capable of detecting the amount of fine particles in exhaust gas in order to directly measure the amount of fine particles in the exhaust gas (unpurified exhaust gas) and to detect the occurrence of a deterioration or defect in the filter. International Application Publication No. WO2009/109688 discloses one type of fine particle detection system that introduces exhaust gas from an exhaust pipe, mixes the exhaust gas with ionized gas containing positive ions so as to charge fine particles in the exhaust gas by the positive ions, discharges the exhaust gas together with the charged fine particles to the exhaust pipe, and then, determines the concentration of the fine particles in the exhaust gas based on an electric current (called “signal current”) flowing according to the amount of the charged fine particles discharged to the exhaust pipe.